Conventionally, an FeRAM is manufactured by depositing a ferroelectric film, such as lead zirconate titanate (PZT), strontium bismuth tantalum oxide (SBT), bismuth lanthanum titanium oxide (BLT) or strontium ruthenium oxide (SRO) on a first, or bottom, planar, electrode film, and forming a second, or top, electrode film over the ferroelectric layer. The second electrode layer and the ferroelectric film are then etched using a reactive ion etch method, after which the first electrode film is etched using a similar method. The result is a number of stacks comprising a first and second electrode film sandwiching the ferroelectric film.
After etching of the individual FeRAM capacitor structure, it is conventional to form a CAP layer of Al2O3 to prevent hydrogen diffusion into the structure. This layer can be formed by a sputtering process. It is important to seek to prevent hydrogen diffusion into the structure as this may damage the device.
In the manufacture of other devices, including semiconductor devices, it is known to form a layer of material such as Al2O3 over the device to prevent hydrogen diffusion. Such diffusion may otherwise affect the properties and performance of the device. Usually an upper insulation layer is also applied.
With such devices, it may be necessary to form an electrical contact with the underlying region or layer, for example with wells in the substrate material, or directly to an underlying layer, for example to a first electrode of an FeRAM. Such an electrical contact maybe formed by forming a contact hole through the overlying layers to an underlying layer or region, and filling the contact hole subsequently with electrically conductive material, such as aluminium. Where the device includes a barrier layer, the contact hole must be formed through the barrier layer to an underlying layer. To ensure low contact resistivity and high reliability, a wet etching treatment is applied. Due to the different etch rates of the different etched layers, steps and/or voids can be formed in the sidewalls of the contact holes. In ferroelectric devices, the barrier or encapsulation (CAP) layers can be pulled or etched back relative to other layers. Next, a thin liner layer is deposited in order to ensure proper filling of conductive material, such as aluminium, into the contact hole. The liner material is usually comprised of Ti or TiN. Even if the liner material is made of a material capable of preventing or reducing hydrogen diffusion, the liner is still ineffective since it is unable to fully fill the steps and/or voids. The resulting voids between the contact liner and the barrier layer creates a leakage path for the hydrogen.
An example of this form of structure is illustrated in FIG. 10, which shows a portion of an FeRAM device is shown. The device includes a ferroelectric capacitor which comprises a bottom electrode 21 and a top electrode 23. Between them is a ferroelectric layer 30. The ferroelectric capacitor is supported within a matrix of TEOS 20. The bottom electrode 21 is connected via an electrically conductive barrier layer 37 of Ir and/or IrO2 to a conductive plug 39 which extends to lower levels of the device. The top electrode 23 is in contact with a top electrode contact well 40, which is a cavity in the TEOS 20, lined with a liner layer of conductive material 47 and filled with a metal 49. Lower layers of the device (not shown) are electrically connected to a conductive element 41. The element 41 extends to a deep contact well 50, which is a cavity in the TEOS 20, having an inner surface coated with a conductive liner layer 43 and being filled with a metal 45. The structur includes a number of barrier layers 31, 33, 35, which may be formed by Al2O3 for example. The purpose of the barrier layers 31, 33, 35 is to inhibit diffusion of oxygen and hydrogen through the structure. Nevertheless, diffusion paths still remain, such as the one indicated by the dashed line 60.
Each of the contact wells 40, 50 is formed by a process having the steps of (i) opening the wells 40, 50, (ii) performing a wet etch process, (iii) coating the wells 40, 50 to form the liner layers 43, 47, and (iv) filling the wells 40, 50 with metal 45, 49.
FIG. 11 is a view of the portion A of FIG. 10. In the formation of the deep contact well, 50, the wet etch process caused the portion of the Al2O3 barrier layer 35 closest to the well 50 to be removed, creating a void 61. The liner 43 does not fill this void 61, so a path indicated by the arrow exists for hydrogen to diffuse around the edge of the barrier layer 35.
FIG. 12 is a view of the portion B of FIG. 10 during the formation of the deep contact well 50 and before the liner 47 is applied. At this stage an annealing operation is being carried out (e.g. to crystallise the PZT layer 30) In an oxygen atmosphere, and O2 is able to diffuse into the TEOS material 20 along the paths indicated by arrows.
FIG. 13 is a view of the portion B after the wet etching step, and after the liner 47 and metal material 43 have been inserted into the top electrode contact well 40. The wet etching step caused the portions of the barrier layers 33, 35 close to the well 49 to be partially removed, creating voids 63 through which hydrogen can diffuse along paths indicated by the arrows.
It would be desirable to form a contact which would not allow undesirable materials to pass across a barrier layer.